Molybdenum in carbon containing niobium-base alloys

ABSTRACT

Niobium-base alloys containing at least 20 weight percent molybdenum, small amounts of carbon, and small amounts of zirconium, hafnium, titanium, or mixtures thereof. Molybdenum prevents the formation of undesirable Nb3C2 and greatly inhibits the formation of undesirable Nb2C during cooling of the alloy after its initial heat treatment (solution annealing). The alloy must contain at least 20 weight percent molybdenum in order for the final product to be substantially free of the undesirable niobium carbides. By preventing the formation of the niobium carbides, the molybdenum causes carbon to be free to react with zirconium, hafnium, titanium, or mixtures thereof and form desirable carbides with those elements during a second heat treatment. The alloys of this invention are useful where strong, lightweight materials are needed such as in the manufacture of jet engine turbines.

United States Patent [72] Inventor Friedrick G. Ostermann Yellow Springs, Ohio [21] Appl. No. 817,559 [22] Filed Apr. 18, 1969 [4S] Patented Sept. 21, 1971 [73] Assignee The United States of America as represented by the Secretary of the Air Force [54] MOLYBDENUM IN CARBON CONTAINING NlOBlUM-BASE ALLOYS 2 Claims, No Drawings [52] 11.8. CI 75/174, 148/32.5, 148/133 [51] Int. Cl C22c 27/00 [50] Field of Search 75/174; 148/32, 32.5, 133

[56] References Cited UNITED STATES PATENTS 2,973,261 2/1961 Frank 75/174 Primary Examiner-Charles N. Lovell Attorneys-Harry A. Herbert, Jr. and Alvin B. Peterson ABSTRACT: Niobium-base alloys containing at least 20 weight percent molybdenum, small amounts of carbon, and small amounts of zirconium, hafnium, titanium, or mixtures thereof. Molybdenum prevents the formation of undesirable Nb -,C and greatly inhibits the formation of undesirable Nb C during cooling of the alloy after its initial heat treatment (solution annealing). The alloy must contain at least 20 weight percent molybdenum in order for the final product to be substantially free of the undesirable niobium carbides. By preventing the formation of the niobium carbides, the molybdenum causes carbon to be free to react with zirconium, hafnium, titanium, or mixtures thereof and form desirable carbides with those elements during a second heat treatment. The alloys of this invention are useful where strong, lightweight materials are needed such as in the manufacture of jet engine turbines.

MOLYBDENUM IN CARBON CONTAINING NIOBIUM- BASE ALLOYS BACKGROUND OF THE INVENTION l. Field of the Invention This invention is in the field of niobium-base alloys.

2. Description of the Prior Art Between about 1955 and 1960 there was some experimentation with niobium-base alloys containing molybdenum, carbon, and zirconium (or acceptable substitutes for zirconium). These alloys were largely developed for application as structural materials for aerospace (reentry) vehicles. Because these alloys had to be workable at room temperature and because large weight percentages of molybdenum decrease the workability at room temperature, molybdenum content was generally held to less than 20 weight percent and was preferably in the range of about to about l5 weight percent.

Around 1960 it was found that, for many purposes, tungsten is a more efficient solid solution strengthener of niobium than molybdenum and most niobium-base alloy work turned to the niobium-tungsten system. However, for certain purposes such as the manufacture of jet engine turbines, the tungsten containing alloys have too high a density to be useful. Therefore, in the recent past, interest in niobium-base alloys containing molybdenum has been rekindled. Also, in such operations as jet engine turbine manufacture, workability of the alloy at room temperature is not always necessary, i.e. turbine blades can be cast or worked at higher temperatures.

The use of carbon in niobium-base alloys has created a problem in the past. The preparation of niobium-base alloys generally involves a heat treatment at about l,700 C., cooling to room temperature, followed by a second heat treatment at about 1,200 to l,300 C. During the cooling step the carbon forms undesirable carbides with niobium which are present in the final product in the form of coarse, unstable needles. Quenching with liquid tin has been used as a means for preventing the formation of large carbide needles. Liquid tin quenching has several drawbacks in that it embrittles the alloy or tends to initiate cracks in the alloy, and is too complicated for large scale production.

SUMMARY OF THE INVENTION It has now been found, quite unexpectedly, that increasing the molybdenum content of niobium-base alloys to weight percentages in the range of from to greatly increases the high temperature strength characteristics of the alloy. It has also been found that increasing the molybdenum content eliminates the need for liquid tin quenching because the larger weight percentages of molybdenum prevent the formation of Nbacg (one of the undesirable niobium carbides) and inhibits the formation of Nb C (a second undesirable carbide). By preventing the formation of undesirable niobium carbides, the large weight percent of molybdenum acts to make carbon available to form desirable zirconium, hafnium, and titanium carbides during the second annealing of the alloy. These carbides are extremely fine and completely dispersed throughout the final product and thus lend strength to the final product.

DESCRIPTION OF THE PREFERRED EMBODIMENT A general discussion of the method used to prepare the alloys of this invention has been given above. It bears repeating that the weight percentage of molybdenum used in the alloy is very critical and that the percentage should be at least 20 and preferably in the range of from about 25 to 30. This will be shown by the following examples.

The following examples serve to completely illustrate the invention and make it understandable and practicable by one skilled in the art. All percentages given are weight percentages unless otherwise stated. The weight percentages of niobium are not given, it being understood that the balance of the alloys over and above the percentages given for the other elements is niobium. Thus, for example, Nb-O. lC means an alloy containing 0.1 weight percent carbon and 99.9 weight percent niobium, and Nb-l5Mo-0.1C means alloy containing 0.1 weight percent carbon, 15 weight percent molybdenum and 84.9 weight percent niobium.

EXAMPLE I Alloys A, B, and C were prepared. Alloy A was Nb-0.lC. Alloy B was Nb-l5Mo-O.1C. Alloy C was Nb-30Mo-0.1C. The elements were placed together and melted. The melting was accomplished by nonconsumable arc melting under a purified argon atmosphere in water-cooled copper molds. The alloys were remelted three times to insure thorough mixing of the ingredient components. Fifty gram buttons were recovered from the molds and were solution annealed for one hour at 1,750 C. and radiation cooled in a conventional vacuum furnace. The approximate cooling rate between 1,750 C. and l,000 C. was 400 C. per minute. Metallographic evaluation showed that grain and subgrain boundaries of all three alloys contained carbide phases. Alloy A and alloy B in addition contained coarse carbides in the grain interior.

The three alloys were then annealed in vacuum at 1,200 C. for an additional 24 hours. This second anneal produced no structural changes in alloy A and very little structural change in alloy B. I-Iowever,'the second anneal produced an extensive precipitation of carbides in alloy C. This additional precipitation was in the form of discontinuous precipitation or growth of carbide plates from the grain or subgrain boundary carbides into the grain interior.

This example illustrates that precipitation of carbidesduring the cooling step in the Nb-30Mo-O.1C alloy (alloy C) is especially sluggish and confined to crystal defects and that carbon supersaturation exists in alloy C after cooling from the solution annealing temperature. This carbon supersaturation apparently does not exit or exists only to a very small degree in alloy B (which contains only 15 weight percent molybdenum) and does not exist at all in alloy A (which contains no molybdenum).

EXAMPLE II Five alloys were prepared in the manner similar to that described by Example I. The alloys were Nb-lZr-0.l3C-l0 Mo, Nb-lZr-0.l3C-l5Nb-lZr-0.l3C-25Mo, and Nb-lZr- 0.13C-3Mo. The elements were melted in copper molds as described by Example I and 50 gram buttons of each composition were recovered. The buttons were then solution annealed at l,900 C. for 5 hours. Vacuum cooling at the rate of Example I was then carried out. A comparison of the microstructures after this solution anneal showed coarse, undesirable niobium carbide needles in alloys with less than 20 weight percent molybdenum which disappeared in the 25 and 30 weigh percent molybdenum containing alloys. Finally, sections of the buttons were given a second annealing treatment for 1 hour at temperatures between 1,000 C. and 1,800 C. This anneal caused the formation of zirconium carbides. The microstructures of samples after this second annealing treatment revealed a unique effect of molybdenum in the concentration range of 20 to 30 weight percent on the size and distribution of zirconium carbides. Whereas alloys containing less than 20 weight percent molybdenum showed comparatively coarse and rather nonuniformly distributed zirconium carbide particles, these carbides were extremely uniformly distributed and of a small and hardly detectable size in alloys with 25 and 30 weight percent molybdenum annealed for 1 hour at 1,400 C. This indicates the excellent high temperature stability and strength achieved by the addition of 25 to 30 weight percent molybdenum.

still going on to decide which of the two, if either, is preferable for use in certain applications, Also, combinations of the three (zirconium, hafnium, and titanium) may be used to yield very good alloys.

The foregoing examples have given precise values for carbon and zirconium weight percentages. Of course it is well known in the art that the weight percentages of carbon may be varied in the range of about 0.05 to about 0.2 and the weight percentages of zirconium may vary within the range of about 0.5 to about 3.0. If hafnium or titanium or combinations of zirconium, hafnium, and titanium are used in lieu of zirconium, the weight added should, of course, be equivalent on the 

2. A niobium-base alloy according to claim 1 wherein the zirconium is replaced by corresponding atomic percentages of a member selected from the group consisting of hafnium, titanium, and combinations of zirconium, titanium, and hafnium. 